Ferromagnetic particles containing chromium



. FER-ROMAGNET PARTICLES CONTAINING CHROMIUM Filed May 23, 1969 IN VENTORS ERNEST LEWIS LITTLE, JR. JACK D. WOLF A ORNEY United States Patent US. Cl. 75.5 7 Claims ABSTRACT OF THE DISCLOSURE Iron alloys with chromium and boron, optionally containing cobalt and/ or nickel, can be made in the form of ferromagnetic particles by reduction of a solution of the appropriate metal salts with an alkali metal borohydride, preferably at an initial temperature between 40 and 100 C. If formed in a magnetic field, the particles chain up to acicular particles of higher coercivity. The particles can be compacted to form permanent magnets and are useful for making recording members such as magnetic tape. Preferred compositions include iron/chromium/boron a1- loys having a very small amount of nickel or cobalt or both which improves the magnetic properties.

FIELD OF THE INVENTION This invention relates to new ferromagnetic compositions and more particularly compositions comprising fine ferromagnetic particles of iron optionally alloyed with nickel and/or cobalt, and containing boron and chromium.

The Miller and Oppegard US. Pat. 3,206,338 describes and claims ferromagnetic alloys of iron with boron and optionally cobalt and/ or nickel in the form of fine acicular particles. Despite the fine particle size of these alloys, they are nonpyrophoric. In accordance with the present invention, it has been discovered that when chromium salts are included in certain reaction mixtures containing iron and reduction is carried out on them with a borohydride, alloys containing metallic chromium in substantial proportions are produced which have improved oxidation stability (compared with the alloys of Oppegard and Miller) and good magnetic properties. This discovery is particularly surprising, since chromium is classified as a reluctant metal which cannot be produced by wet chemical reduction of its salts.

SUMMARY OF THE INVENTION The novel ferromagnetic alloy particles of the present invention have cross-sectional dimensions of 0.01 to 0.3 microns, a length of 0.01 to 4 microns and preferably an axial ratio of at least 3 :1; consisting essentially by weight of up to 35% cobalt, up to 35% nickel, and from 0.4 to 20%, preferably from 8 to 20%, chromium, and 1.0 to 7.5% boron dissolved therein, and oxygen either as metal oxide, metal hydroxide or as moisture balance Fe in an amount of at least 30%.

Especially preferred are iron alloys containing from 8 to 12% by weight of chromium, from 1.0 to 7.5 by weight of boron, and from about 0.1 to by weight of at least one of cobalt and nickel.

The compositions are produced by the reduction, in solution, of a mixture of the appropriate metal salts with an alkali-metal borohydride or alkaline earth metal borohydride and, if elongated particles are desired, preferably in the presence of a magnetic field of at least oe. and more preferably at least 1000 oe.

The magnetic particles of the present invention, preferably in elongated or acicular form, can be made into magnetic objects. Thus, they can be compacted to form useful permanent magnets or other magnetic objects with or without a binding material. Any binding material used can be a thermosetting or thermoplastic organic polymeric binder or an air-drying, film-forming binder.

The finely particulate material of the present invention can also be used in magnetic recording members such as magnetic recording tapes.

DETAILED DESCRIPTION OF THE INVENTION The reduction process by which the novel magnetic materials of the present invention are made is generally conducted in aqueous solution, but alcohol, tetrahydrofuran, or like organic solvents can also be used as a reaction medium.

A wide variety of metal salts can be used including the halides, sulfates and nitrates. Salts of organic acids, e.g., acetic acid or stearic acid, can also be used. It is preferred that the metals be in low valence state (compatible with the requirement of solubility) in order to reduce the amount of the borohydride compounds needed for the reduction.

The preferred ionic borohydride compounds for use in the process of this invention are sodium and potassium borohydrides. Other alkali metal and alkaline earth metal borohydrides such as lithium borohydride, magnesium borohydride and calcium borohydride are operable, but are less readily available. While the proportion of metal borohydride and metal salts can be Varied considerably, the preferred proportion is about one mole of metal borohydride to two moles of metal salts.

The pressure at which the reaction is carried out is normally atmospheric pressure, but higher or lower pressures can be employed if desired.

Temperature has a pronounced effect on the process. Ambient temperature is operable, but it has been found that the efficiency with which chromium metal is incorporated into the composition of this invention increases with temperature up to about 40 C. Above 40 C. the efliciency is substantially constant with temperature. The proportion of boron incorporated decreases steadily with increasing temperature, but to less extent than the variation of chromium incorporation with increasing temperature. Heat is liberated in the reduction process, and it is therefore difficult to maintain the process temperature constant, particularly when large batches are to be processed. Accordingly, it is preferred to conduct the reduction process at a minimum initial temperature of about 40 C. Higher temperatures can be employed, but increasing temperature increases the loss of valuable borohydride reagent by the catalytic action of the fine metallic particles produced. Accordingly, it is preferred to operate at temperatures less than 100 C.

The metal salts are dissolved in water or other solvent to form a strong solution, preferably saturated. The borohydride reagent is also dissolved to form a strong solution, which is then added to the solution of metal salts.

The reagent solutions are mixed together in a relatively brief period which is suitably from 2 seconds to 30 min utes, the relatively longer times in this range being used for larger-scale preparations. Ten minutes is a generally useful time over which to add the borohydride reagent solution to the solution of metal salts. The reaction mixture should be stirred during the addition of the reagents to effect mixing, but excessive stirring hinders the formation of acicular particles, and if these are desired, excessive stirring should be avoided. A rotating magnet can be employed to assist mixing and to provide a'magnetic field.

As indicated above, the reaction can be conducted in the presence of a magnetic field of at least 100 cc. and preferably of at least 1000 oe. in order to promote the formation of acicular particles. A suitable method of performing the reaction is to employ a non-magnetic reaction vessel, which can be glass, ceramic or stainless steel to contain the reactants, which is placed in the field of a permanent magnet or an electromagnet.

After the reduction process, the particles are washed immediately with water, acetone or the like and then dried.

Structural analysis by X-ray diffraction and electron diffraction of the Fe-Cr-B powder shown in Examples 2, 4 and 5 indicates that the principal phase present is the body centered cubic structure of a-iron, the reflection being somewhat diffuse. No extra reflection that could be attributed to a boride or boron oxide is present. Since the solubility of boron in iron at room temperature is less than 0.01% by weight [M. Hansen, Constitution of Binary Alloys, McGraw-Hill, p. 251, 1958], the compositions of the present invention are solid solutions of metallic chromium in u-iron, supersaturated with boron.

Elemental analysis indicates that the composition of the present invention contains a minor amount of oxygen, which in view of the above consideration and the very high surface/volume ratio of the particles is in part attributable to oxidation of the surface, and, in part, to absorption of water or other oxygenated solvent on the surface of the particles.

The transverse dimensions of the particles vary from about 0.01 to 0.3 microns, and the lengths are about 0.01 to 4 microns. The axial ratio of the majority of the elongated particles is at least 3 to 1 and may be as much as 100:1 or higher.

When produced in the absence of a magnetic field, the products contain a high percentage of equiaxed particles, the size of the particles being dependent on the particular composition. With Fe/Cr/B compositions the particles are generally in the range of 0.05 to 0.08/L. Small amounts of cobalt and/or nickel decrease the particle size and increase the coercivity. The percentage of iron or nickel which provides a useful effect depends on the concentra tion of other materials present. With from 8 to 12% Cr and 1 to 7% B by weight, the compositions should contain from about 0.1 to about 5% and preferably from 0.5 to 2% by weight of at least one of nickel and cobalt to obtain enhanced magnetic properties.

In the presence of a magnetic field, elongated particles are obtained which appear to be chains of the equiaxed particles. These elongated particles can be described as acicular, although when viewed using the electron microscope, the surface of these particles is undulating rather than smooth.

The particles show substantially improved oxidation stability over iron or iron alloy particles which do not contain chromium. For optimum stability, the chromium content should be at least 5% by weight. The preferred compositions of the present invention contain from about 8 to 20% by weight of chromium.

The particles can be compacted by the techniques known in the field of powder metallurgy to form useful permanent magnets. Optionally, small quantities of organic or inorganic binder can be present, generally from about 2 to 30% by weight, based on the total composition of binder, is employed. High percentages of binder can be used, but are not generally desirable, since the binder is generally inert magnetically.

In the powder form the particles can be mixed with a film-forming binder and coated as a suitable substrate to form a magnetic recording member. A common form of a magnetic recording member, magnetic tape, is shown in FIGS. 1 and 2 of the appended drawings.

FIG. 1 shows a plan view of a magnetic tape. FIG. 2 shows a cross-section of the tape along the line AA. In the figures, a substrate (1) is provided, which is generally a flexible polymeric film having suitable mechanical properties, i.e., it should be flexible as noted above, dimensionally stable with time, and under stress. Suitable polymeric film supports include films of poly(ethylene terephthalate) which has been oriented by stretching biaxially, cellulose acetate and like materials. A coating of ferromagnetic particles. in a binder (2) is coated onto the surface of the supporting film and calendered to a smooth, even layer. In addition to conventional magnetic tapes employed for magnetic recording, the ferromagnetic particles of the present invention can be used to manufacture other recording members such as the patterned recording members employed for reflex thermomagnetic imaging as taught by Nacci, Belgian Pat. 627,017 in which the magnetic material in a film-forming binder is printed in the form of a halftone pattern on a substantially transparent substrate, or filled into indentations or grooves embossed in the surface of a transparent supporting member.

EMBODIMENTS OF THE INVENTION This invention is further illustrated by the following examples, which should not, however, be constnled as fully delineating the scope of this discovery. In these examples, parts and percentages are by Weight unless otherwise specified. The horseshoe magnets used in the examples had field strengths of 1500-1700 oe. Magnetic properties were determined by packing the powders in tubes and placing them in an extraction magnetometer with an applied field of about 4400 oe. Saturation magnetization values, a and remanent magnetization values, o' are given in the examples as emu/g.

EXAMPLE 1 A solution of 43.5 g. of FeSO '7H O and 20 g. of Cr (SO -xH O (3035% H O by weight) in 500 ml. of distilled water was prepared, and 7.6 g. of NaBH was dissolved in 250 ml. of distilled Water. The NaBH solution was slowly added to the metal salts solution. A vigorous, exothermic reaction took place and a black, magnetic solid separated. This solid was thoroughly washed with water and then with acetone. It was allowed to air dry. The product weighed 6 g. It contained 59.85% Fe, 16.95% Cr. 5.32% B, balance oxygen and water. The product had an intrinsic coercive force, ,H,,, of 253 oe., a saturation magnetization, 0' of 41.8 emu/g. and a remanence ratio (F /0' of 0.258.

EXAMPLE 2 A solution of 27.8 g. of FeSO -7H O and 2.5 g. of K Cr (SO -24H O in 200 ml. of distilled water was put in a 2-liter beaker resting on the poles of a horseshoe magnet having a field of 1500 0e. A solution of 3.8 g. of NaBH in ml. of cold, distilled water was slowly added over a ten-minute period. The black precipitate Was separated by filtration and washed with water and then acetone. It was suspended in ml. of acetone for 16 hours before filtering and air drying. The product weighed 4 g. It contained 66.86% Fe, 6.82% Cr, 3.06% B, balance oxygen and water. The product had an intrinsic coercive force, H of 417 oe., a saturation magnetization, a of 96 emu/g. and a remanence ratio (er 0) of 0.302, Particle size: Av. diam.: ca. 0.064 av. length: ca. 1

EXAMPLES 315 Examples 3-6 illustrates the preparation of products containing various proportions of iron and chromium. The general procedure was as described in Example 2. In all cases a solution of 3.8 g. of NaBH in 100 ml. of cold, distilled water was used. The amounts of iron and chromium salts and analytical data on products are summarized in Table I.

The magnetic properties of the above products are summarized in Table II.

TABLE 11 Example 111., a. Ur r/ 's As indicated in the general definition of the products above, the oxygen present in the particles of this invention can be in the form of metal oxides and/ or hydroxides or as moisture. The amount of moisture in a product made by the method of Example 3 was found to be 3.02%. This moisture content was determined as follows: The Weight loss of a sample heated in a vacuum of 0.1 micron was measured on a Du Pont 950 Thermo-gravimetric Analyzer. The sample was heated from room temperature to 400 C. at a rate of l/minute. The sample lost 3.02% of its original weight between room temperature and 175 C., with no additional loss to 400 C.

Examples 7-15 illustrate the eifect of temperature on the product using the same quantities of material and the same general procedure except that the reaction vessel was maintained in a constant temperature bath at the stated temperatures and in a field of 1700 oe. The results are given in Table III.

TABLE III p., Cr, 13, 0. percent percent 4151., a. o,- (Yr/0' The morphologies of the powders were determined by the use of an electron microscope at a magnification of 20,000X. The particles synthesized below 40 C. Were about 50% acicular with average dimensions of about 1.0 x 0.08 1. The particles synthesized at 40 C. or greater were about 8% acicular with typical dimensions of 1.6 x 0.054

EXAMPLES 16-19 The magnetic properties of these same materials are summarized in Table V.

1 5 TABLE V Example 4H5 a. at 4/ 5 EXAMPLES 20-24 These examples illustrate the preparation of IFe-Cr-B products containing various proportions of cobalt. The general procedure was as described in Example 2. In all cases, a solution of 3.8 g. of NaBH in 100 ml. of cold, distilled water was used. The amounts of iron and chromium salts were 44. 6 g. of FeSO -7H O and 20 g. of K Cr (SO -24H O. The amount of cobalt sulfate used and analytical data for the products are summarized in Table VI.

TABLE VI Approx. avg. dimensions, Percent microns Fe Co Cr B Diam. Length A solution of 44.6 g. of FeSO -7H O, 10.5 g. of NiSO -6H O and l g. of K Cr (SO' -24H O in 200 ml. of distilled water was put in a 2-liter beaker resting on the poles of a horseshoe magnet of field strength 1500 oe. A solution of 3.8 g. of NaBH in 100 ml. of cold, distilled water Was slowly added over a ten-minute period. The black precipitate was filtered and Washed with water and then acetone. It was suspended in 125 ml. of acetone for 16 hours before filtering and air drying. The product Weighed 4.1 g. It contained 48.57% Fe, 18.11% Ni, 1.75% Cr, 3.48% B, balance oxygen and water. The 65 product had an intrinsic coercive force, H of 1040 oe.,

TABLE IV Approx. avg. dimensions, Percent microns K2C12(SO4)4-24H2O Example (g.) Fe Co Cr B Diam. Length a saturation magnetization per gram, a of 66 emu/ g. and a remanence ratio, (T of 0.425.

EXAMPLE 26 An iron-nickel-chromium-boron alloy powder was synthesized in the presence of a magnetic field of about 1700 oe. A 2-liter beaker resting on the poles of a horseshoe magnet was charged with a solution containing 52.8 g. of FeSO -7H O, 2.6 g. of NiSO -6H O, and 1 g. of K Cr (SO -24H O dissolved in 200 ml. of distilled water. A solution of 3.8 g. of NaBH in 100 ml. of distilled water was slowly added to the beaker in a l0-min. period. During the addition of the NaBH solution, a vi orous exothermic reaction occurred forming a black magnetic powder. The powder product was filtered, washed with water, and washed with acetone. After washing, the product was suspended in acetone for about 16 hr. before final filtration and air drying.

The chemical composition of the powder was 73.78% Fe, 1.27% Ni, 4.10% Cr and 2.14% B. It had a (T of 106 emu/g, an H of 1155 oe., and a a' /o' ratio of 0.47. The powder consisted essentially of acicular particles with an average width of about 0.04 1. and an average length of about 0.6 When a powder product is prepared by the same procedure as described for this example in the absence of an external magnetic field, equiaxed particles of about 0.03 in diameter are formed.

EXAMPLE 27 An iron-cobalt-nickel-chromium=boron alloy powder was synthesized in the presence of a magnetic field of about 1700 oe. A 2-liter beaker resting on the poles of a horseshoe magnet was charged with a solution containing 50.0 g. of FeSO -7H O, 2.8 g. of CoSO -7H O, 2.6 g. of NiSO -6H O, and 1 g. of K Cr (SO -24H O dissolved in 200 ml. of distilled water. A solution of 3.8 g. of NaBl-L; in 100 ml. of distilled water was slowly added to the beaker in a 10-min. period. During the addition of the NaBH solution, a vigorous exothermic reaction occurred forming a black magnetic powder. The product was filtered, washed with water and washed with acetone. After washing, the product was suspended in acetone for about 16 hr. before final filtration and air drying.

coated magnetic stirring bar was attached to the tip of the buret in such a way that the tip rotated in phase with the rotating horseshoe magnet. A black solid separated as the addition proceeded. Occasional external stirring was necessary to disperse the bulky solid product. After the addition was complete, the mixture was filtered, and the solid product was washed with two liters of distilled water and one liter of acetone and allowed to stand in acetone for 16 hours. It was separated by filtration and air dried. The yield was 13.0 g. The product contained 76.82% Fe, 0.15% Co, and 7.77% Cr, the balance being boron, oxygen and water. It had an intrinsic coercive force, H of 520 oe., a saturation magnetization, a of 109 emu/g., a remanence magnetization, 0 of 41 emu/g, and a remanence ratio, er /(r of 0.376. It consisted of about 75% acicular particles, with average diameter 0.05 and average length 072.

EXAMPLE 29 The general procedure of Example 28 was employed. A solution of 278 g. of FeSO -7H O, 50 g. of K2CI'2(SO4)424H2O and g. of in 1 litCr of distilled water was prepared and a solution of 19 g. of NaBH in 500 ml. of cold, distilled Water was slowly added. The precipitate formed was collected and washed as previously described. The product contained 71.0% Fe, 8.4% Cr, 3.1% Co, and 2.29% B, the balance being oxygen, hydrogen and water. The particles had an average width of 0.04 micron. The measured magnetic properties were H of 710 oe., a of 114 emu/g, a, of 43 emu/g. and o' /a' of 0.377. The results of this experiment in comparison with Example 5 show that a small amount of cobalt incorporated in an iron/chromium/boron alloy increases the coercivity of the particles.

EXAMPLES 30-35 The general procedure employed with these examples was as described for Example 2. In all cases a solution of 3.8 g. of NaBH in 100 ml. of cold water was employed. The amounts of the other reactants are summarized in Table VIII. The analytical data of the products is given in Table IX and the magnetic properties of the products are given in Table X.

TABLE VIII FGSO4- KzC1 (SO4)4- COSO4- N1304- Example 71320, 1;. 241120, g. 7H2O fiHzO Yield, g.

The chemical composition of the powder was 66.56% Fe, 3.47% Co, 1.22% Ni, 5.04% Cr, and 2.11% B. TABLEIX It had a a of 92 emu/g, an H of 1190 oe., and a Percent; (T /0' ratio of 0.48. The powder consisted essentially of E 1 F C C acicular particles with an average width of about 0.04/L xamve e r o 1 W1 and an average length of about 1,5. When a powder 8'8; product is prepared by the same procedure as described 8:6 217 1'5 0: 05 for this example in the absence of an external magnetic 8 38% field, equiaxed particles of about 0.03 1. in diameter are 1219 0I5 0I5 0:03 formed.

EXAMPLE 28 F 50 7H 0 (278 K Cr (SO 24H 0 (50 g) 6 TABLE X e 4' 2 2 2 4 4' 2 l and CoSO -7H O (2 g.) were dissolved in distilled water, emu/g emu/g and the solution was made up to a volume of one liter. gig g; 33 8.11 1; The solution Was placed in a constant-temperature bath 765 88 37 0:420 set at C., and a 1700-0e. horseshoe magnet capable 23 g 8%; of being rotated was placed below the assembly. With 810 3 42 01480 the horseshoe magnet rotating at about 15 r.p.m., 500 ml. of a solution of 19 g. of NaBH in distilled water was added to the solution of metal salts at a rate of 25 ml. per minute from a buret with a flexible tip. A small plastic- As pointed out above, the chromium-containing alloys of the invention are more resistant to oxidation than similar alloys containing no chromium. The enhanced oxidation resistance is illustrated as follows:

An Fe-B powder and Fe-Cr-B powders containing 2.3, 3.7, 5.7, 7.1 and 11.0% of Cr were prepared by the method of Example 2. A sample of each product was immersed in 30% by weight of nitric acid at 25 C. for 3 minutes, and another sample of each product was immersed for 10 minutes. Each mixture was filtered, and solid was washed with water and dried to a constant weight in a vacuum oven. The weight percentage of each powder that had dissolved was calculated. The results are summarized in Table XI and show clearly the beneficial effect of increasing chromium content. The Fe-B powders reacted violently and completely with the nitric acid, while the powder containing 11.0% Cr showed no visible reaction with the acid for 3 minutes.

TABLE XI Exposure to 30 wt. percent HNO percent Fe+percent Cr 3 min. 10 min.

Percent Cr The stability towards oxidation by 30% nitric acid is related to the stability of metals in moist air (The Corrosion Handbook, Ed. Uhlig, John Wiley & Sons, New York, N.Y., 1948, p. 28).

As also pointed out above, the alloys of this invention are useful in magnetic applications, e.g., as magnets and in magnetic tapes. These utilities are illustrated in Examples A and B, which follow.

EXAMPLE A An iron-chromium-boron powder prepared as described in Example 1 was formed into a permanent magnet as follows: A l-gram sample of the powder was pressed in a 1.5" x 0.1" mold at 80,000 p.s.i. and room temperature. The resulting bar was magnetized by placing it in a magnetic field. The bar had a coercive force, H of 335 oe., a saturation magnetization per gram, of 74.2 emu/g. and a remanence ratio of 0.414.

EXAMPLE B A blend of several batches of Fe-Cr-B powder synthesized by the technique described in Example 2 above was fabricated into magnetic recording tapes. The powder blend was comminuted for 5 hours with poly(tetrafluoroethylene) balls in a rotating plastic canister to increase its apparent bulk density from about 0.1 to 1.0 g./cc. The powder blend had an H of 459 oe., a of 98 emu/g., and a o of 36 emu/ g. The average chemical composition of the powder was 71.6% Fe, 9.6% Cr, 1.5% B, and 14.2% 0.

After comminution, the powder was ground with 20- to 30-mesh sand in a slurry of tetrahydrofuran plus soya lecithin for 1 hour and then mixed in the sand grinder with a binder consisting of 50%, by weight, of a soluble polyester-urethane resin made from diisocyanatodiphenylmethane, adipic acid and butanediol, and 50% of vinylidene chloride/acrylonitrile /20 copolymer. The binderpowder system contained about 30 volume percent of powder. After mixing, the binder-powder slurry was pressure filtered through a 2, screen to remove the sand. Coatings of the filtered slurry were then spread on a 1.5-mil. thick film of poly(ethylene terephthalate). The coated films, which were about 3 inches wide and 30 inches long, were passed between the poles of two plate magnets that created a field of about 800 oe. parallel to long direction of the tape. The tape was then dried in air for about 24 hours and dried in a vacuum desiccator for about 16 hours. Measurements of properties of the Fe-Cr-B magnetic tape are summarized in Table XII.

TABLE XII.--PROPERTIES OF Fe-Cr-B MAGNETIC TAPE /2" max. 1.285 Coating thickness, mils a- 0.260 B,, g. 1,530 12,02 0.70 H.;, oe 440 The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. Ferromagnetic alloy particles having cross-sectional dimensions of 0.01 to 0.3 micron and a length of 0.01 to 4 microns consisting essentially of, by weight, up to 35% cobalt, up to 35% nickel, 0.4% to 20% chromium, 1.0 to 7.5% of boron dissolved therein, oxygen either as metal oxide, metal hydroxide or moisture, and the balance iron in an amount of at least 30%.

2. Composition of claim 1 in which the axial ratio of said particles is at least 3:1.

3. Composition of claim 2 in which chromium is present in an amount of from 8 to 20% by weight.

4. A magnetic recording member containing, as the magnetic material, the particles of claim 3.

5. A magnet formed from the particles of claim 3.

6. Composition of claim 3 in which chromium is present in an amount of from 8 to 12% together with from 0.1 to 5% by Weight of at least one of nickel and cobalt.

7. Composition of claim 5 in which said nickel and cobalt are present in an amount of from 0.5 to 2% by weight.

References Cited UNITED STATES PATENTS 3,206,338 9/1965 Miller et a]. 75-.5

HYLAND BIZOT, Primary Examiner W. W. STALLARD, Assistant Examiner U.S. Cl. X.R. 148-3155, 

